LECTURE 27: DEQUES CSC 212 – Data Structures

Roses are red and violets are blue Implement push, pop, & top And you’re a Stack too! Stack & ADT Memory Aid

public interface Stack { public E top() throws EmptyStackException; public E pop() throws EmptyStackException; public void push(E elem); public int size(); public boolean isEmpty(); } Stack Interface

Queue Memory Aid It’s hard writing rhymes with enqueue, dequeue, and front

public interface Queue { public E front() throws EmptyQueueException; public E dequeue() throws EmptyQueueException; public void enqueue(E elem); public int size(); public boolean isEmpty(); } Queue ADT

Stack vs. Queue  Access data with Stack in LIFO order  LIFO  L ast I n- F irst O ut is totally unfair (unless always late) ‏  Data accessed in Queue using FIFO order  FIFO  F irst I n- F irst O ut ensures early bird gets the worm Order read if Stack

 Cannot access both sides of Collection  Transplant waiting lists  Help center phone banks  Grandpa dealing cards for money  Stack only works on one end  All insertions & removals from the Stack ’s top  Queue limits how each side used  Elements can only be added to end  Front allows accessing & removal of elements Still Have Limits

 Pronounced “deck” (like on a house) ‏ DEQUE  Mnemonic for Double Ended QUEue  dequeue ≠ deque and do not sound alike  Structure that provides access to both ends  Combines Stack & Queue concepts  Is also able to add elements to start Deque ADT

public interface Deque { public E getFirst() /* front() */ throws EmptyDequeException; public E removeFirst() /* dequeue() */ throws EmptyDequeException; public E getLast() /* top() */ throws EmptyDequeException; public E removeLast() /* pop() */ throws EmptyDequeException; public void addFirst(E elem); /* Brand new! */ public void addLast(E elem); /* push() & enqueue() */ public int size(); public boolean isEmpty(); } Deque Interface

VIP Lines & Lucky Losers

 Class defines fields aliased to first & last nodes  Doubly-linked list enables O(1) time for all methods  Add elements by adding new Node at end  Update sentinels next/previous to remove element Linked-list based Deque head rear Ø retVal Ø

 DEQUES, like QUEUES, have both ends move  addFirst & removeFirst moves its front  Structure’s end moved by addLast & removeLast  Ends of a array-based DEQUE like clock time  Identical to Queue and how it works, except…  …occasionally need to subtract from index, also Circular Access q r f r rrr r r r f

Array-based DEQUE Operations  Uses property of clock math  Remainder of result is all that matters  But the values sign is also important  -1 % 4 == -1, for example  To get this to work, use a cheap trick  Adding size of the array does not change result (-1 + 6) % 6 (3 + 6) % 6 = 5 % 6= 9 % 6 = 5= 3

 Start week #10 assignment  Due by 5PM next Tuesday  Continue programming assignment #3  Messages are not always sent to everyone!  Read section 6.1 in book before class  What if we wanted to access all the items?  Could we get something like a resizable array?  How do ranks differ from indices? Before Next Lecture…